1. Field of the Invention
This invention relates to magnetic bubble devices, and, more particularly, to certain substituted iron garnet compositions for use in those devices.
2. Description of the Prior Art
A magnetic bubble memory consists of a thin film of magnetic garnet or other magnetic material in which microscopic cylindrical magnetic domains may be generated and moved. The axes of the domains are normal to the film surface; thus, when viewed end on (using polarized light) the domains have the appearance of small disks or "bubbles." In operation, the film is maintained in a bias field directed normal to the film. The magnitude of the bias field is kept within the range over which the bubbles are stable. At the lower limit of that range, the "strip-out field," the bubbles grow until they distort into elongated strips. At the upper limit, the bubbles collapse. Controlled perturbations of the magnitude and direction of the magnetic field near the bubbles are used to move the bubbles. To provide the greatest operating latitude, the bias field is set in the middle of the stable range, providing a characteristic bubble diameter. The smaller the bubble diameter, the greater the amount of information that can be stored in a particular area.
The diameter, d, of a magnetic bubble domain can be related to the characteristic length parameter, l EQU l=(AK.sub.u).sup.1/2 /.pi.M.sub.s.sup.2
where A is the magnetic exchange constant, K.sub.u is the uniaxial magnetic anisotropy constant, and M.sub.s is the saturation magnetization. Nominal bubble diameter is d=8l. Magnetization, as seen, plays an important role in determining the bubble size. Iron garnets such as (Y,Sm).sub.3 Fe.sub.5 O.sub.12 have a magnetization too high to support stable bubbles near 1.5 .mu.m diameter. Ge, Al, Ga, or another element is often substituted for Fe on the tetrahedral crystal site in these iron garnets to reduce the net magnetic moment of the iron sublattices and thereby the magnetization of the garnet bubble material.
One deleterious side effect of such a substitution is that the Curie temperature, the temperature at which the magnetization drops precipitously to nearly zero, is decreased. For example, it has been noted (U.S. Pat. No. 3,886,533) that Ga substitution for Fe results in a substantial lowering of the Curie temperature. The region of large change in magnetization with temperature, which is near the Curie temperature, is thus reduced to near the operating temperature range of a magnetic bubble memory device. A large temperature variation of the magnetization prevents the usual method of temperature stabilization of bubble memory devices; that is, adjustment of the temperature variation of the magnetic properties of the bubble material, principally the bubble collapse field, to about that of the temperature variation of the magnetization of the biasing magnet (U.S. Pat. No. 3,711,841).
(Ca,Ge)-substituted iron garnet compositions of the (La,Lu,Sm,Ca).sub.3 (Fe,Ge).sub.5 O.sub.12 system were studied for use as "small bubble materials" by S. L. Blank et al., J. Appl. Phys. 50, 2155 (1979). Within that system, they identified a composition that is suitable as a 1.1 .mu.m bubble material. However, that composition has limited usefulness, because the temperature coefficient of the bubble collapse field (.alpha..sub.bc) is too large.
In a series of patents issued to Blank (U.S. Pat. Nos. 4,002,803; 4,034,358; and 4,165,410), iron garnet systems using (Ca,Sr)- and (Ge,Si)-substitution for iron were disclosed, including various compositions that are suitable for layers capable of supporting stable magnetic bubbles. Among the compositions are ones that contain rare earth elements such as thulium (Tm) in octahedral sites in a relative molar concentration of from 0.01 to 0.1 per formula unit. Over a temperature range, the bubble collapse field for these compositions is claimed to vary with temperature at approximately the same average rate as the bias field variation with temperature over that range. U.S. Pat. No. 4,034,358 discloses a composition of the type R.sub.3-a (Ca,Sr).sub.a Ge.sub.a Fe.sub.5-a O.sub.12 where R is yttrium or a rare earth element having atomic number between 57 and 71.